This invention concerns the production of unsaturated esters by oxidation of olefins with molecular oxygen in the presence of at least one carboxylic acid in homogeneous phase, the catalyst being an organo-metallic complex of palladium containing two identical or different hydroxamate anions or one hydroxamate anion and one carboxylate anion.
The invention provides in particular for the conversion of ethylene to vinyl acetate according to the equation: EQU CH.sub.2 .dbd.CH.sub.2 +CH.sub.3 CO.sub.2 H+1/2O.sub.2 .fwdarw.CH.sub.2 .dbd.CH--OCOCH.sub.3 +H.sub.2 O
or the conversion of propylene to allyl acetate according to the equation: EQU CH.sub.3 --CH.dbd.CH.sub.2 +CH.sub.3 CO.sub.2 H+1/2O.sub.2 .fwdarw.CH.sub.2 .dbd.CH--CH.sub.2 --OCOCH.sub.3 +H.sub.2 O
or the conversion of isobutene to methallyl acetate according to the equation: ##STR1##
The presently existing processes which operate in homogeneous liquid phase and provide for the conversion of ethylene to vinyl acetate, generally make use of an acetic solution containing alkali ions, copper ions, and acetate ions and chlorine ions as well as bivalent palladium salts. This technique has been used in processes described in many patents, particularly in the British Pat. No. 964,001 and the U.S. Pat. No. 3,260,739.
The chlorine ions have a decisive action for regenerating bivalent palladium and prevent its precipitation from the medium as palladium metal. However these catalytic solutions containing chlorine ions in acetic medium are responsible for a substantial corrosion of stainless steel and the formation of chlorinated products which are difficult to remove. On the other hand, as stated in the U.S. Pat. No. 3,906,036, a substantial portion of copper is converted to copper oxalate which is insoluble in the acetic medium and is catalytically inactive.
Catalytic systems operating in the absence of halide ions have been suggested, particularly in the U.S. Pat. No. 3,238,247, but they have only a very limited life time.
Most of the existing processes providing for the conversion of ethylene to vinyl acetate are operated in heterogeneous phase in the presence of a catalyst containing palladium deposited on an inorganic carrier such as silica or alumina and in the presence of promoters such as alkali or alkaline earth metal salts of carboxylic acids. This is described, for example, in the U.S. Pat. No. 3,822,308. However, these processes in heterogeneous phase are conducted at high temperature and produce a non-negligible amount of CO.sub.2. Moreover, they yield a relatively low production and require a periodic regeneration of the catalyst.
One of the advantages of the process according to the present invention is to proceed in the liquid phase in the complete absence of halide ions. Another advantage of the process consists in the fact that it provides, for example, for the production of vinyl acetate from ethylene and of allyl acetate from propylene with high velocity and selectivity under moderate temperature conditions.